Firearms such as handguns, rifles and the like are commonly manufactured with some form of safety, but such a safety is not, in itself, generally sufficient in preventing or deterring unauthorized use of a firearm. lt may, accordingly, be expedient to have an additional security device, which can be applied to a firearm to prevent its unauthorized use (and possibly serve as a deterrent against theft of the firearm) but which can be readily removed by an authorized user. The present invention provides such a device.